The test, likely to cost between £100 and £300, can show with almost 90 per cent accuracy which individuals suffering from mild memory loss are going to develop Alzheimer’s within a year.

It is expected to transform the search for treatments for the devastating brain illness that affects around 600,000 people a year in the UK.

To date, trials of drugs to halt or reverse Alzheimer’s have all ended in failure. Therapies exist that can reduce its symptoms, but they only work for a short period of time and are not very effective.

Scientists believe a major reason for the lack of progress is that trial patients are being recruited too late, when their disease is already far advanced.

The new blood test, based on 10 “biomarker” proteins, will make it possible to test new treatments at an early stage of Alzheimer’s progression.

It could also help families plan ahead and adjust to the likely prospect of one of their members being stricken by the disease.

A step in the right direction

There has been so little progress in tackling Alzheimer's disease, any new development is met with enthusiasm, writes Channel 4 News Science Editor Tom Clarke.

But a test - even an imperfect one like this - has researchers particularly excited because it's something the Alzheimer's field has badly needed for a long time. An effective test could be the key to treating Alzheimer's.

Several high profile drugs which effectively target the cellular processes thought to be behind Alzheimer's have been through clinical trials in recent years. Yet all have failed.

One of the best explanations researchers have for this is that the drugs are only given to volunteers once they have been diagnosed with the disease. Because Alzheimer's symptoms only begin once brain damage is quite advanced, the thinking is the drugs are simply being given too late to make any difference.

This latest test can distinguish between those with the common memory loss associated with ageing and those who will go on to develop Alzheimer's disease. While it's not reliable enough for GPs to start using tomorrow, having a rapid blood-based test for Alzheimer's could soon be used to improve trials of many interesting drugs in development (or already developed) to see if they can forestall the devastating symptoms of the disease.

The "holy grail" for Alzheimer's researchers will be a test that can predict who will suffer Alzheimer's before any symptoms emerge and, hopefully, before any damage occurs to the brain. This test isn't that, but it's a step in the right direction.

Symptoms of memory loss

One of the test’s inventors Professor Simon Lovestone, from King’s College, London, said: “People come to me at the clinic because they want to know what’s happening to them, and I currently can’t tell them. I tell them: ‘You’ve got symptoms of memory loss – come back in a year’s time.’

“That’s grim. It’s horrible. You can only imagine what it’s like for the patient.”

Writing in the journal Alzheimer’s & Dementia, the researchers describe how they investigated 26 proteins previously associated with Alzheimer’s disease.

People come to me at the clinic because they want to know what’s happening to them, and I currently can’t tell them.Prof Simon Lovestone

In the vast majority of cases, memory lapses do not lead to Alzheimer’s. But the researchers identified 10 blood proteins that appeared in 87 per cent of MCI patients diagnosed with the disease within a year.

Commercial interest

Talks are underway with several potential partners who might be interested in producing a commercial product.

Dr Ian Pike, a director of Proteome Sciences and co-author of the research, said: “By linking the best British academic and commercial research, this landmark study in Alzheimer’s disease is a major advance in the development of a simple blood test to identify the disease before clinical symptoms appear.

“This is the window that will offer the best chance of successful treatment.”

We have to be very careful about how we use these tests, especially in the absence of effective therapy.Dr Eric Karran

Dr Eric Karran, science director at the charity Alzheimer’s Research UK, said a test identifying those at risk of Alzheimer’s at an early stage would be of “real value” but warned that it would have to be used responsibly.

“Alzheimer’s disease is now the most feared diagnosis,” he said. “We have to be very careful about how we use these tests, especially in the absence of effective therapy.”

In practice the blood test is not expected to be used on its own, but as a first step in a diagnostic process that could also involve brain scans and taking samples of spinal fluid.